Among nonvolatile semiconductor memories, MRAM (Magnetoresistive Random Access Memory) which is a memory utilizing changes in magnetic resistance and SPRAM (Spin Transfer Torque RAM) can perform fast operations, and at the same time, they can function as a nonvolatile RAM that can practically and infinitely reprogrammable (Japanese Patent Application Laid-Open Publication No. 2006-179125 and Japanese Patent Application Laid-Open Publication No. 2002-093149 (Patent Documents 1 and 2)).
As illustrated in an equivalent circuit diagram of FIG. 26A and a schematic cross-sectional view of FIG. 26B, the SPRAM is formed of a tunnel magnetoresistive element TMR, a select transistor MCT, a word line WL, a bit line BL, and a source line SL. Further, as illustrated in FIG. 27, the tunnel magnetoresistive element TMR has at least two magnetic layers. One of these layers is a fixed layer PL in which the direction of spin is fixed, and the other is a free layer FL that takes two states in which the directions of spin are parallel and anti-parallel to the fixed layer PL. A tunnel barrier film TB formed of MgO or the like is provided between these magnetic layers.
Storage of information is performed according to the directions of spin of the free layer FL. That is, in the anti-parallel state to the fixed layer PL of FIG. 27A, the electric resistance of the tunnel magnetoresistive element TMR is set in a high resistance state, and, in the parallel state of FIG. 27B, the electric resistance is set in a low resistance state. Therefore, these states are assigned to “0” and “1” of the information. In the reading operation, magnitude of the resistance of the tunnel magnetoresistive element TMR is read, thereby obtaining the stored information. In the rewriting operation, a current is applied vertically to the tunnel barrier film TB and the free layer FL so that the direction of spin of the free layer FL is controlled. That is, when a current is caused to flow in the direction toward the free layer FL from the fixed layer PL, electrons having the direction of spin that makes the direction of magnetization of this free layer FL opposite to the direction of the fixed layer PL mainly flow into the free layer FL. Consequently, when this current value exceeds a fixed threshold value, the directions of the magnetization of the fixed layer PL and the free layer FL become anti-parallel. In contrast, when a current is caused to flow toward the fixed layer PL from the free layer FL, electrons having the direction of spin that makes the direction of the magnetization of this free layer FL same as the direction of the fixed layer PL mainly flow into the free layer FL. When this current value exceeds a fixed threshold value, the directions of the magnetization of the fixed layer PL and the free layer FL become parallel. That is, in this memory cell, the information “0” and “1” is distinguished in writing by the directions of current. When this system is employed, the current (threshold value) required for rewriting the information is made proportional to magnitude of the tunnel magnetoresistive element TMR. Consequently, the memory cell can be miniaturized along with reduction of the rewriting current, and thus scalability becomes excellent.